1. Field of the Invention
The present invention relates to a disc playback apparatus, and more particularly, to an optical pickup for use in playing back a multi-layer disc and a multi-layer disc playback apparatus for reproducing recorded information from the multi-layer disc.
2. Description of the Related Art
A Digital Video Disc (DVD) is an optical disc capable of storing a large amount of digital information. It is capable of storing digital information, such as animation and computer information, in a disc of the same 12 cm diameter as a compact disc (CD); at a recording density 6-8 times greater than a CD. Various devices are used in recording onto a DVD to achieve such high density recording. For example, the DVD uses a laser having a wavelength of 650 nm or 635 nm, shorter than a wavelength of 780 nm used for a CD, to increase the recording capacity. The numeral aperture, NA, of an objective lens is 0.6, larger than the NA of 0.45 for a CD. As a result, a high density recording of about 5 G byte has been achieved on a single side of the disc.
Because the DVD is of higher density recording than a CD, the spot diameter of a laser beam for reproducing bit information needs to be far smaller than that of the CD. The spot diameter of a laser beam is proportional to the wavelength .lambda. of the laser beam and inversely proportional to the numeral aperture NA of an objective lens. In the DVD, this is realized by using a laser beam having a short wavelength and an objective lens having a large numeral aperture NA.
Further, by using a multi-layer disc composed of a plurality of recording layers instead of a single layer recording disc, a further high-density disc has been realized. In such a multi-layer disc, the focal point of the light beam is set to a desired recording layer for recording or reproduction.
FIG. 6 illustrates light beam behavior due to wave front aberration when data in a multilayer disc is reproduced by a single pickup. An objective lens 602a of the pickup illustrates the position thereof when a first recording layer 601a of the multi-layer disc 610 is being played back. An objective lens 602b illustrates the position thereof when the n-th layer, which is far from the disc substrate surface 601s is being played back.
In a pickup optimally designed for focusing a light beam on the first layer, there is no trouble in playing back the first layer. However, if the objective lens approaches the disc substrate surface 601s to play back the n-th layer and the light beam is focused on the n-th layer, wave front aberration occurs due to the thickness of layers between the first layer and the n-th layer. As a result, as shown in FIG. 6, the diameter of the beam spot irradiated onto the n-th layer is larger than that on the first layer.
In the multi-layer disc, a distance from the recording layer optimally designed for the optical pickup creates a difference in the diameter of the beam spot actually focused on the layer. As a result, wave front aberration (mainly spherical aberration) occurs, and each recording layer has a different wave front aberration. Particularly, a recording layer far from the recording layer optimally designed is more affected by the wave front aberration.
That is, when a light beam is gathered onto a recording layer other than the optimally designed recording layer for the optical pickup, the light beam cannot be focused due to an influence of the aforementioned wave front aberration. Thus, in recording layers other than the optimally designed recording layer, it is necessary to enlarge the pit size or pit interval. Accordingly, the recording density per layer drops. As a result, the recording density of the overall multi-layer disc cannot be raised as much as desired.